Method of producing a photo-electric tube



July 10, 1934. H. c. RENTSCHLER El" AL 1,966,220

METHOD PRODUCING A PHOTO ELECTRIC TUBE Original Fi led Feb. 28, 19 9INVENTORS H. c. REA/7.50945? 11.5 HE/V ATTORN EY Fatented July 1Q, 1934;

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METHOD OF PRODUCING A PHOTO-ELEC- ERIC TUBE Harvey C. Rentschler, EastOrange, and Donald E. Henry, Bloomfield, N. 5., assignors toWestinghouse Lamp Gompany, a corporation of Pennsylvania Olaims.

This application is a division of application Serial Number 343,532filed February 28, 1929 by the present applicants and assigned tothesame assignee as the present application.

5 The invention relates to a photo-electric tube of the type employingas the photo-sensitive material, an oxide or sub-oxide of an alkalimetal, such as caesium.

One form of photo-electric tube to which my invention particularlyrelates comprises an envelope of glass containing a cathode orphotosensitive electrode having a surface of silver, with a film of anoxide or sub-oxide of an alkali metal, such as caesium thereon. It hasbeen found that when the surface of the cathode is provided with such afilm of caesium-oxide or sub-oxide, the sensitivity of the tube to lightin the red and yellow region of the spectrum is increased several fold.It is essential, however, in order to maintain the sensitivity of thetube in this region, that there be no free caesium or other alkali metalin the envelope.

The method which has been employed, heretofore, in producingphoto-electric tubes of this material is briefly as follows:

The cathode, which may consist of a plate of silver or silver platedcopper, is mounted in the envelope with an anode and after exhaustion ofthe envelope oxygen is admitted and a discharge created between theanode and cathode to oxidize the silver. The oxygen is then exhaustedand the alkali metal introduced. The bulb and contents are then heatedup to effect a reaction between a portion of the caesium and the silveroxide to form a film of caesium oxide or sub-oxide, on the cathode. Theheating is continued for a sufficiently long period and temperature tovaporize the excess free caesium out of the bulb through the exhaustsystem. A Difiiculty has been experienced with this procedure due to thehigh temperature required to vaporize all the caesium out of theenvelope. This must be done with great care and in view of this hightemperature baking it is necessary to employ an all glass manifold onthe pumping system.

The process is slow and tedious and not well suited for factoryproduction, wherein it is desirable to employ the regular rubber exhaustconnections. This heating may also break down some of the activecaesium-sub-oxide and result in a tube of low sensitivity.

One of the objects of the present invention is to produce a simplifiedprocess of manufacturing a photo-electric tube of this nature.

Another object of the invention is to provide a novel means of removingfree caesium from a photo-electric tube employing, as the lightsensitive material, a compoundof caesium.

Other objects and advantages will hereinafter appear.

In accordance with the invention we provide within the envelope a gettersubstance which is inert or at least less active with respect to thealkali metal than is the oxidized silver surface of the cathode, at thereaction temperature of the silver oxide and alkali metal, but whichreacts with the alkali metal at a slightly higher temperature to formsolid, stable compounds thereof. The reaction temperature of the getterwith the alkali metal should not be so high as to cause the m cathode tobe heated to the dissociation temperature of the oxide or sub-oxide ofthe alkali metal when the getter is heated to such reaction temperature.

By the use of such a getter material the tube, after oxidation of thesilver surface of the cathode, may be exhausted, the alkali metalgenerated, the gas filling, if any, introduced and the tube permanentlysealed prior to the production of the film of the sub-oxide of thealkali metal. The film of alkali metal oxide or sub-oxide may beproduced on the cathode by heating the Whole tube to a temperature atwhich the alkali metal reacts with the silver oxide. The excess freealkali metal may then be cleaned-up by heating the getter to 35 thereaction temperature thereof with such metal while the bulb and othercontained parts are heated to a sufiicient temperature to prevent thealkali metal from depositing thereon.

The getter preferably consists of a quantity 90 of lead glassconstituting a portion of the envelope although a compound may beemployed either as a coating on some portion of the bulb or itscontents, or in a suitable container or capsule. Other methods ofintroducing the getter will be apparent, a number of which are disclosedin the accompanying drawing.

Reference is had to the drawing for a more complete understanding of theinvention.

Fig. 1 is an elevation of a tube embodying my invention, showing theenvelope in section, in which a portion of the glass envelopeconstitutes the getter material.

Figs. 2, 3, 4 and 5 are fragmentary views of a 1% photoelectric tubeshowing modified methods of utilizing the getter material.

Referring to Fig. l, a photo-electric tube is shown comprising anenvelope 6 having the upper or body portion 7 formed of lime glass orother glass not attacked by caesium vapor and a lower or neck portion 8formed of lead glass.

The envelope contains a plate cathode 9 which in cross section may be ofV-shape or semi-circular and which conveniently may consist of silver orsilver plated copper. The cathode 9.is supported from the press 10 bysupport wires 11 and 12, the latter of which is joined to a leadinginconductor 13. An anode, in the form of a wire 14, which may be ofnickel, is positioned within the concavity of the cathode, and issupported above the press by supports 15 and 16, the former of which isconnected to a leading-in wire 17.

A capsule 18 is supported above the electrode assembly by a wire 19welded to the upper end of the anode. The capsule contains a chemicalmixture capable, when heated, of generating caesium vapor in the device.

A mixture which we prefer to employ in the capsule 18 consists ofcaesium-dichromate and silicon, the silicon serving as a reducing agentfor the caesium-dichromate. A small proportion of a metal such as mischmetal or aluminum may be added to the mixture to expedite the reactionand to decrease the reaction temperature.

The method of producing the tube is as follows: After the electrodes aresealed into the envelope, the tube is baked out and exhausted throughthe exhaust connection 20 in the usual manner. After the bulb has cooledit is filled with oxygen at a pressure of about 1.6 mm. and a directcurrent discharge is passed between the silver plated copper, as cathodeand the wire anode, using about 800 to 1000 volts and a low current ofabout to 100 milliamperes. This discharge is continued until the properdegree of oxidation has been obtained, which may require only a veryshort time.

The preferred degree of oxidation is obtained when the plate color showsa green color, at least in spots. The oxygen is then pumped out and thecapsule 18 heated by high frequency. induction current to generate thecaesium metal in the tube, and if it is desired to employ a gaseousatmosphere in the tube,- argon or other gas is introduced at the properpressure. The tube is then sealed off.

All of the above operations may be conducted on the usual tube exhaustsystem using rubber connections to the exhaust manifold. The bulb isnext placed in an oven and heated to about 125 C. for about 5 minutes tocause a reaction between the caesium vapor and the oxidized silver,producing probably a sub-oxide of caesium. At this temperature there isno appreciable reaction between the caesium and the lead glass portionof the envelope so that all the caesium is available for forming thesub-oxide film on the cathode.

The excess caesium is then cleaned up by heating the lead glass portionin a second adjacent oven at about 350 to 400 C. at which temperaturethe caesium reacts with the lead oxide content of the lead glass,forming stable solid caesium compounds. During this treatment the limeglass portion of the tube is maintained at a temperature of about 125 C.to prevent condensation of the caesium in the lime glass portion of thetube and to drive it over to the lead glass portion. Higher heating ofthe lime glass portion should be avoided in order not to destroy orimpair the caseium oxide or sub-oxide film on the cathode. This heatingoperation should be continued until the free caesium is entirely cleanedup but ordinarily 5 to 20 minutes will be sufficient.

The bulb is then thoroughly cooled and a reading taken to determine thephoto sensitivity in the red and yellow portions of the spectrum. If theemission is below normal, the entire bulb may be again heated for about3 minutes at 200 to 250 C. generally, the occasional tubes which do nothave the normal photo-electric sensitivity after the first treatmentwill be greatly improved by this latter treatment.

It is absolutely essential to remove all the free caesium from the tubeby this clean-up action, since small quantities of free caesium if leftin the tube deposit on the cathode and destroy the sensitivity of thetube in the red and yellow region, and tends to cause electrical leakagebetween the leading-in wires.

After the tubes have been heat treated they may be seasoned under normaloperating conditions for several hours in order to remove all traces ofresidual gases in the envelope other than the monatomic gases.

In Fig. 2 we have indicated the getter as consisting of a coating 21applied to the interior of the neck portion of the bulb. In this casethe entire bulb may be constructed of lime glass. A compound which maybe used is lead oxide applied to the bulb before sealing in the mount.

In Fig. 3 the getter is shown as consisting of a ring of oxidized copper22 supported on the stem tube 23 and capable of being heated by highfrequency induction currents. With this construction the getter may beheated independently of the cathode so that the danger of reducing thecaesium sub-oxide film during clean up of the excess caesium, iseliminated.

During heating of the getter ring 22 the bulb should be baked to asufiicient temperature to vaporize the free caesium from all parts ofthe device. -In place of using an oxidized copper surface for the gettermaterial suitable compounds capable of reacting with the caesium may beapplied to the ring 22 and heated by induction.

We have shown, in Fig. 4, a getter capsule 24 suspended from one of theelectrode supports in a position to be conveniently heated by highfrequency induction currents. This capsule may contain a material suchas copper oxide, or spongy lead oxide which when heated above thetemperature required to form the caesium sub-oxide film on the cathode,cleans up the excess caesium.

Fig. 5 shows the getter material in the form of a coil 25 of oxidizedcopper, having its opposite ends connected to the cathode supports byconductors 26 and 27. A separate leading-in conductor 28 and 29 isprovided for each of the cathode supports so that the coil 25 may beheated to the reaction temperature by passing electric current directlytherethrough to prevent short circuiting of the coil through thecathode, one of the cathode supports is provided with an insulating head26' intermediate the press and the cathode. Any suitable getter materialmay also be supported within the coil 25 as to be heated thereby.

In employing the various forms of getters illustrated the caesiumsub-oxide film is first produced on the cathode and the getter thenactivated to clean up the excess caesium under such conditions that thecaesium sub-oxide film is not decomposed.

It is obvious that changes may be made in the embodiments describedwithout departing from the invention and we desire to include all suchchanges within the scope of the invention.

What is claimed is:

1. The steps in the method of producing a light sensitive device havinga film 01' an oxide of an alkali metal on a metallic cathode, comprisingoxidizing the surface of the cathode, introducing an alkali metal intothe envelope, heating the oathode to effect a reaction between saidalkali metal and the oxidized cathode and cleaning up all of the excessfree alkali metal by chemical action at a temperature above thatnecessary to cause said reaction between the alkali metal and theoxidized cathode.

2. The steps in the method of producing alight sensitive device havingan oxide of an alkali metal on a metallic cathode, comprising oxidizingthe surface of the cathode, introducing caesium metal into the envelope,heating the cathode to the reaction temperature of the alkali metal withthe metallic oxide and cleaning up all of the excess free caesium withlead oxide.

3. The steps in the method of producing a light sensitive device havinga film of caesium suboxide on a metallic cathode, comprising forming theenvelope in part only of lead glass, oxidizing the surface of thecathode, introducing caesium into the envelope, heating the envelope toa temperature sufliciently high to cause a reaction between the caesiumand the oxidized cathode, and then increasing the temperature of thelead glass portion of the envelope to the reaction temperature of thecaesium with the lead glass while maintaining the remainder of theenvelope at a lower temperature.

4. The steps in the method of producing a photo-electric tube having afilm of caesium suboxide on a metallic cathode, comprising forming theenvelope in part only of lead, glass, oxidizing the surface of thecathode, producing caesium in the envelope, heating the envelope toabout 125 C. to cause a reaction between the caesium and the oxidizedcathode and then heating lead glass portion only of the envelope toabout 350 to 400 C. to cause a reaction between the excess caesium andthe lead glass, while maintaining the remainder of the envelope at about125 C.

5. The steps in the method of producing a photo-electric tube having afilm of an alkali metal compound on a metallic cathode comprisingoxidizing the surface of said cathode, generating an alkali metal in theenvelope, heating the cathode to effect a reaction between the alkalimetal and the oxidized cathode and subsequently activating a getterwhile maintaining the envelope at an elevated temperature and bychemical action cleaning up all of the excess alkali metal.

6. The steps in the method of producing a photo-electric tube having afilm of caesium suboxide on a metallic cathode, comprising forming theenvelope in part only of lead glass, oxidizing the surface of thecathode, introducing a quantity of caesium therein, heating-the envelopeto about 125 C. for about five minutes to cause a reaction between thecaesium and the oxidized cathode to iform a film of caesium sub-oxide onthe cathode, and subsequently heating the lead glass portion of theenvelope at about 350 to 400 C. for 5 to 20 minutes to clean-up theexcess free caesium, while maintaining the remainder of the envelope ata temperature of about 125 C.

and rendering excess alkali metal therein ineffec- 7. The steps in themethod of producing a lightsensitive device including an enclosingenvelope,

which comprise forming a deposit of a lightsensitive material in saidenvelope and by chemical action rendering ineffective the excessphotosensitive material contained therein.

8. The steps in the method of producing a lightsensitive deviceincluding a container, which comprise depositing a layer of an alkalimetal therein tive by chemical action.

9. The steps in the method of producing a lightsensitive deviceincluding a container, which comprise depositing a layer of an alkalimetal therein and rendering ineifective excess alkali metal therein by achemical action between said excess alkali metal and another substancehaving a component thereof whose temperature of vaporization is greaterthan that of said alkali metal.

10. The steps in the method of producing a light-sensitive deviceincluding a container, which comprise depositing a layer of alkali metalon an oxidized surface located in said container, oxidizingsaid alkalimetal and reacting excess free metal with a metal oxide to render saidfree excess alkali metal ineffective.

11. The steps in the method of producing a light-sensitive deviceincluding a container which comprise depositing a layer of caesiumtherein by reducing a non-hygroscopic compound of caesium therein andrendering excess caesium ineffective by chemical action.

12. The steps in the method of producing a light-sensitive deviceincluding a container which comprise depositing a layer of caesiumtherein by flashing an admixture of caesium dichromate and silicon toliberate caesium and by chemical action rendering ineffective excessfree caesium contained therein.

13. The steps in the method of producing a light-sensitive device,including a container which comprise depositing a layer of caesium ontoan oxidized surface, converting the caesium to a sub-oxide and renderingineffective the excess free caesium by chemical action between the sameand a compound that is stable at the temperature of said sub-oxideformation.

14. The steps in the method of producing a light-sensitive deviceincluding a metallic base layer, comprising oxidizing the surface of thebase, introducing an alkali metal into the envelope, heating the base toeffect a reaction between said alkali metal and the oxidized base, andcleaning up the excess free caesium by chemical action.

1 5. The steps in the method of producing a light-sensitive device,including an oxide of an alkali metal on a metallic cathode, comprisingoxidizing the surface of the cathode, introducing 185 an alkali metalinto the envelope, heating the cathode to the reaction temperature ofthe alkali metal with the metallic oxide and cleaning up the excess freecaesium with lead oxide.

HARVEY C. RENTSCHLER. DONALD E. HENRY.

